1. Field of the Invention
The present invention relates to a process for preparing a semiconductor device. More particularly, it relates to an improvement of a process for forming an aluminum-containing film on a nitride film formed by a plasma enhanced chemical vapor deposition (plasma CVD nitride film).
2. Description of the Prior Arts
The prior arts of the present invention will be described for one embodiment of a preparation of a multi-layer wiring film for a large scale integrated circuit (LSI).
FIG. 1 shows a sectional view of only wiring parts of the conventional LSI after forming a pattern of a two layer wiring. In FIG. 1, the reference numeral (1) designates a silicon substrate (Si); (2) designates an oxidized film (SiO.sub.2); (3) designates a first layer of an aluminum wiring film; (4) designates a plasma CVD nitride film; (5) designates a through-hole; (6) designates a second layer of an aluminum wiring film.
After forming the first layer of the aluminum wiring film (3), the plasma CVD nitride film (4) is formed and the through-hole (5) is formed and then, the second layer of the aluminum wiring film (6) is formed on the plasma CVD nitride film (4) and the through-hole by a metal vapor deposition, etc. In such structure, there is large possibility for a failure caused by a short-circuit between the first layer of the aluminum wiring film (3) and the second layer of the aluminum wiring film (6). This is caused by a local structural defect such as pin-holes formed in the plasma CVD nitride film or a short-circuit between both the aluminum wiring films (3) and (6) which is resulted by partially reacting the plasma CVD nitride film (4) with the second layer of the aluminum wiring film (6) as shown as A in FIG. 1. This has been confirmed by the following tests.
The first layer of the aluminum film (3) is formed on the silicon substrate (1), and the plasma CVD nitride film (4) is formed on the aluminum film (3). The second layer of the aluminum film (6) is formed on the nitride film to prepare a sample. The sample is heat-treated in a nitrogen atmosphere at 500.degree. C. Variations of element distribution in the sample to the depth direction before and after the heat-treatment are measured by an auger electron spectroscopy.
FIG. 2 shows the results of the auger electron spectroscopy wherein depths from the surface of the second layer of the aluminum film (6) is plotted on the abscissa and auger electron intensities as relative contents of the elements in the depths are plotted on the ordinate. The full line shows the distribution before the heat-treatment and the broken line shows the distribution after the heat-treatment. In FIG. 2, the reference (a) corresponds to the oxygen; (b) corresponds to aluminum in the second layer of the aluminum film (6); (c) corresponds to nitrogen in the plasma CVD nitride film (4); (d) corresponds to oxygen; (e) corresponds to aluminum corresponding to the first layer of the aluminum film (3); (b') and (e') respectively show variations after the heat-treatment which correspond to the curves (b) and (e).
This result shows the fact that the aluminum in the second layer of the aluminum film (6) is easily diffused into the plasma CVD nitride film (4) by the heat-treatment. On the other hand, the first layer of the aluminum film (3) is not substantially diffused into the plasma CVD nitride film (4). This reason is considered that the alumina layer formed naturally in the surface layer of the first layer of the aluminum film (3) imparts the effect for preventing the diffusion, since the peak for oxygen is found in an interface between the first layer of the aluminum film (3) and the plasma CVD nitride film (4). After forming the first layer of the aluminum film (3), the aluminum layer is exposed to the atmosphere whereby the aluminum reacts with oxygen in the atmosphere to form the alumina layer. On the other hand, the second layer of the aluminum film (6) is formed by a vacuum metal vapor deposition in the formation. Therefore oxygen is not incorporated in the interface between the second layer and the plasma CVD nitride film. This is the reason why the diffusion is resulted. The diffused layer reaches to the first film of the aluminum film (3) to cause the short-circuit.